A thermoplastic polymer advanceable by solid state polymerization is blended with at least one dissimilar thermoplastic polymer, or an organic or inorganic particulate filler. The blend is solid state polymerized to provide a modified polymer alloy or filled polymer blend having at least one physical or chemical property different from that of the blend before solid state polymerization. A substrate is coated with an adherent layer of the modified polymer alloy or filled polymer blend. The modified polymer alloy or filled polymer blend may also be coextruded with a layer of thermoplastic extrusion polymer having a melt viscosity similar to that of the modified polymer alloy or filled polymer blend and applied to a substrate to form an adherent coating.